1. Field of the Invention
The present invention relates to a radio frequency (RF) circuit. More particularly, the present invention relates to an RF circuit component that can be used effectively for transmitting, demultiplexing, multiplexing, radiating or detecting an RF signal belonging to the microwave or millimeter wave band, and also relates to an RF circuit including such a circuit component.
2. Description of the Related Art
A waveguide is known as one of various transmission elements for an RF circuit. A waveguide is usually a structure made of a hollow tubular conductor in which electromagnetic fields of certain modes are formed in an internal space surrounded with the conductor. The waveguide allows the electromagnetic waves having a particular frequency to propagate. Examples of waveguides include rectangular waveguides having a rectangular cross section, and circular waveguides having a circular cross section, perpendicularly to the electromagnetic wave propagating direction (see Wiley-Interscience (John Wiley & Sons, Inc.), “Microwave Solid State Circuit Design”, pp. 28–33.).
A typical structure for a rectangular waveguide will be described with reference to FIG. 12. The waveguide shown in FIG. 12 has a rectangular cross section, which has a vertical size of a mm and a horizontal size of b mm (where a<b). Every electromagnetic wave, having an effective wavelength that is at most twice as long as the horizontal size b, can transmit through the inside of this waveguide. However, no electromagnetic wave, having an effective wavelength that is more than twice as long as the horizontal size b, can transmit through it. In other words, the effective wavelength of the electromagnetic wave that can transmit through this waveguide is 2×b mm or less. The velocity c of the electromagnetic wave is represented by effective wavelength×frequency. Thus, the cutoff frequency fc is given by c/(2×b). As a result, electromagnetic waves, of which the frequencies are equal to or lower than the cutoff frequency fc, are cut off.
A rectangular waveguide may also be used as an antenna. FIG. 13 shows a structure for a rectangular waveguide that functions as an antenna. The waveguide shown in FIG. 13 includes an input portion 31 at one end thereof and an aperture plane 32 at the other end thereof. An electromagnetic wave with a predetermined frequency is input through the input portion 31, transmitted through the inside of the waveguide, and then radiated into a free space through the aperture plane 32. In this case, a frequency corresponding to an effective wavelength of 2×b, which is twice as long as the horizontal size b of the input portion 31, becomes the cutoff frequency fc. Accordingly, the antenna shown in FIG. 13 can radiate or receive an electromagnetic wave having a frequency exceeding this cutoff frequency fc.
To realize desired radiation directivity, the horizontal size b1 and vertical size a1 of the aperture plane 32 may be respectively different from the horizontal size b and vertical size a of the input portion 31.
A slot antenna is known as an antenna, of which the structure is similar to that of the rectangular waveguide antenna shown in FIG. 13. FIG. 14(a) is a perspective view of a slot antenna structure, and FIG. 14(b) is a cross-sectional view thereof as viewed on the plane 26.
The slot antenna structure shown in FIG. 14 includes a dielectric substrate 21 with a grounded conductor layer 23 provided on its back surface. A strip-shaped slot 24 is cut through a center portion of the grounded conductor layer 23. The slot 24 is formed by removing a conductor portion of the grounded conductor layer 23 all through its thickness in its own designated area. On the surface of the dielectric substrate 21, a signal conductor line 22 is arranged so as to cross the slot 24 of the grounded conductor layer 23. A microstrip line is defined by this signal conductor line 22 and the grounded conductor layer 23 such that an electromagnetic wave propagates through the microstrip line. In this case, resonance is caused at an effective wavelength that is twice as long as the horizontal width of the slot 24. When the resonance is set up, an electromagnetic wave is radiated through the slot 24 into the free space under the back surface of the dielectric substrate 21. Only an electromagnetic wave, having a frequency close to the frequency at which resonance is caused by the slot 24 (i.e., the resonant frequency), is radiated efficiently into the free space.
A waveguide is used not just as an antenna but also as an RF circuit in various other applications. Japanese Patent Application Laid-Open Publication No. 62-186602 and Japanese Patent Application Laid-Open Publication No. 63-269802 disclose bandpass filters including a waveguide as one of its elements.
As described above, the frequency of an electromagnetic wave that a waveguide can transmit is higher than the cutoff frequency fc. For example, to make a waveguide that passes an electromagnetic wave at 2 GHz, the horizontal size b of the waveguide needs to be at least equal to 7.5 cm. This is because a waveguide with a horizontal width shorter than 7.5 cm would have a cutoff frequency fc higher than 2 GHz and a 2 GHz electromagnetic wave could not be transmitted through the waveguide. That is why if one tried to use such a waveguide in an RF circuit to operate in a frequency range of around 2.4 GHz, then its size would be too big, which is a problem.
However, if a waveguide is loaded with a material with a high dielectric constant, then the cutoff frequency fc of the waveguide can be reduced and the size of the waveguide can also be reduced accordingly.
Hereinafter, the cutoff frequency fc of the waveguide will be described in further detail with reference to FIGS. 15(a) and 15(b). FIG. 15(a) is a graph schematically showing how the transmission intensity of a waveguide, including the air inside, changes with the frequency. On the other hand, FIG. 15(b) is a graph schematically showing how the transmission intensity of a waveguide, which is loaded with a high dielectric material, changes with the frequency.
As can be seen from FIGS. 15(a) and 15(b), no electromagnetic waves can be transmitted at frequencies lower than the cutoff frequency fc. It can also be seen that the cutoff frequency fc can be reduced by loading the waveguide with the high dielectric material. The cutoff frequency fc is inversely proportional to the 0.5th power of the dielectric constant. Accordingly, if the waveguide is loaded with a high dielectric material with a dielectric constant of 9, for example, then the cutoff frequency fc can be reduced to one-third (=1/90.5=⅓). This means that by loading the waveguide with the high dielectric material with a dielectric constant of 9, the effective wavelength of the electromagnetic wave inside the waveguide 1 shortens to one-third.
However, even if a waveguide with a horizontal size b of 3 mm is loaded with such a high dielectric material with a dielectric constant of 9, the cutoff frequency fc can be just reduced from 50 GHz to 16.7 GHz and no electromagnetic waves with a frequency of about 2 GHz can be transmitted, either. To transmit an electromagnetic wave with a frequency of about 2 GHz, the horizontal size b needs to be further increased about eightfold. The same statement applies to an antenna or a slot antenna using a waveguide.
Consequently, as long as the conventional waveguide structure is adopted, even a waveguide with as small a horizontal size as 10 mm or less could not transmit an electromagnetic wave with a frequency of 5 GHz or less.
Each of Japanese Patent Application Laid-Open Publication Nos. 62-186602 and 63-269802 discloses that by arranging a dielectric resonator inside a waveguide, the waveguide can also function as a bandpass filter. However, as schematically shown in FIG. 15(c), the frequency range in which the transmission intensity is increased by the action of the dielectric resonator is still higher than the reduced cutoff frequency fc shown in FIG. 15(b). That is why even if the conventional technique disclosed in Japanese Patent Application Laid-Open Publication Nos. 62-186602 or 63-269802 is used, the size of the waveguide cannot be further reduced compared to the situation where the waveguide is fully loaded with a high dielectric material.